Solution mass density can be used to monitor the condition of a chemical solution which changes as it is used in a process. In some instances, the density of the solution should be maintained within fairly precise limits. For example, in the case of ammoniacal etchant solutions for copper, it is desirable to maintain etchant density to within 0.1% of a set value. An alarm should be provided to warn the operator that the process is out of control if the density varies more than 0.25% from the set value.
A hydrometer float is commonly used to provide a simple, stable indicator of mass density. Variation in the float height changes with the density of the liquid displaced by the float. The float height variation can be monitored electronically and used to generate control signals for automatically replenishing the solution and/or alarm signals.
In many instances, the solution being measured is corrosive, and contains entrained or dissolved gases. The solution can roughen and opacify common plastic materials by attack and deposition. Instruments which rely upon mechanical movement of the hydrometer float along a vertical guide or about a pivot may be prone to error due to increasing friction as deposits accumulate on the guides or pivots. Also, the electronics which sense the position of the float must be isolated from the corrosive solution and fumes. The isolating means may introduce unacceptable errors if it imposes excessive uncontrolled forces on the float, the vertical guides, or pivot.